TL;DR. The Relative Strength Index (RSI) is a bounded momentum oscillator that reads from 0 to 100 and asks a single question: over the last N bars, how much of the total price movement has been to the upside versus the downside? J. Welles Wilder published it in 1978, set the default lookback at 14 periods, and drew “overbought” and “oversold” reference lines at 70 and 30. Those levels are not buy or sell signals. In strong trends RSI can spend weeks camped above 70 or below 30 without reversing, and the indicator’s most durable use — divergence between RSI and price — only becomes visible once you stop treating band touches as automatic reversals.
What RSI actually measures
RSI belongs to the family of momentum oscillators — indicators that convert recent price behaviour into a bounded number so you can compare momentum across time and across securities. Unlike a moving average, which follows price on the same y-axis, RSI lives in its own 0-to-100 pane below the chart. The reading is a ratio: the average size of “up” closes over a rolling window versus the average size of “down” closes over the same window, scaled to sit between zero and one hundred.
The indicator was introduced by J. Welles Wilder in his 1978 book New Concepts in Technical Trading Systems and in the June 1978 issue of Commodities magazine (Wikipedia: Relative Strength Index). It has stayed on default charting layouts at retail brokers ever since, which is a good reason to understand exactly what it is — and what it is not — before drawing conclusions from a 72 reading.
The SEC’s investor-education arm reminds retail investors that technical indicators analyse past price patterns and are not, on their own, forecasts of future returns (SEC Investor.gov). RSI is a description of momentum, not a prediction of what price does next.
The formula, step by step
RSI is computed in three short steps on a rolling window of N bars (Wilder’s default is N = 14):
- Step 1 — split each bar’s change into an up-move and a down-move. For each bar, let U = max(Close − Prior Close, 0) and D = max(Prior Close − Close, 0). Only one of the two is non-zero on any given bar.
- Step 2 — smooth each series. Compute a smoothed moving average of U and of D over N periods. Wilder’s original smoothing (sometimes called RMA or SMMA) is: today’s value = (prior smoothed value × (N − 1) + today’s raw value) / N. Call the results AvgU and AvgD.
- Step 3 — convert the ratio to a 0–100 scale. RS = AvgU / AvgD. Then RSI = 100 − (100 / (1 + RS)).
Fidelity’s technical-indicator guide states the closed form the same way: “RSI = 100 − [100 / (1 + (Average of Upward Price Change / Average of Downward Price Change))]” (Fidelity Learning Center).
Two useful properties fall out of this construction. First, RSI is bounded: if there were no down bars in the window, AvgD = 0, RS is undefined but by convention RSI is set to 100; if there were no up bars, RSI = 0. Second, RSI = 50 corresponds exactly to AvgU = AvgD, i.e. the up-move budget over the last N bars equalled the down-move budget. That 50 line is the practical “neutral momentum” reference.
A worked example
Suppose that over the last 14 daily closes on a stock, the average size of the up-days was $0.80 and the average size of the down-days was $0.40. Then:
- RS = 0.80 / 0.40 = 2.0
- RSI = 100 − (100 / (1 + 2.0)) = 100 − 33.33 = 66.7
Now say the next bar is a big down day that lifts AvgD from $0.40 to $0.55 while AvgU ticks down to $0.75. Then RS = 0.75 / 0.55 ≈ 1.36 and RSI ≈ 100 − (100 / 2.36) ≈ 57.7. A single ugly close is enough to knock nearly nine points off a mid-range RSI, which is precisely why traders watch the direction and slope of the line, not just the level.
Reading RSI: 70 and 30 aren’t magic
Wilder’s original thresholds — overbought above 70, oversold below 30 — have persisted because they are simple and because they roughly bracket where a mean-reverting equity tends to slow down. Fidelity’s own guide notes that these traditional levels “can also be adjusted if necessary to better fit the security” (Fidelity Learning Center). More important is what Fidelity spells out next: during uptrends RSI tends to stay in the 40–90 zone, with the 40–50 area acting as support; during downtrends it stays in the 10–60 zone, with 50–60 acting as resistance.
That single observation dissolves the biggest retail mistake with RSI: assuming that “overbought” means “sell.” A stock in a strong uptrend can print RSI above 70 for weeks and reverse only from a lower RSI reading later on. The overbought line is not an exit trigger — it is a description of momentum intensity.
Divergence: RSI’s most durable signal
The reading traders return to most often is not the level but the divergence between RSI and price:
- Bearish divergence — price makes a higher high, but RSI makes a lower high. The rally is still going, but momentum behind it is fading. It does not predict the top, but it does say the buyer intensity that fueled the last leg has weakened.
- Bullish divergence — price makes a lower low, but RSI makes a higher low. Selling pressure has weakened even though price has drifted lower. Again, not a bottom call; a shift in momentum quality.
Fidelity describes the same idea using Wilder’s original nomenclature of “swing failures”: a “top swing failure” is an RSI lower high followed by a break of the prior RSI low; a “bottom swing failure” is the mirror image (Fidelity Learning Center).
Parameter choices: what changes when you change 14
Traders sometimes lower the lookback to make RSI more responsive or lengthen it to filter noise. Both trade-offs are worth understanding. Wilder’s 14 is a round default, not a mathematical constant; it happens to smooth about three trading weeks of daily data.
| Preset | Period (N) | Behaviour | Typical use |
|---|---|---|---|
| Wilder default | 14 | Balanced sensitivity; canonical 70/30 bands | Daily equity charts, general use |
| Short lookback | 7–9 | More whipsaw; crosses 70/30 more often | Intraday and short-swing setups |
| Long lookback | 21–25 | Smoother; overbought/oversold much rarer | Position trading, weekly context |
| Constance Brown extension | 14 | Uses 80/20 in strong uptrends, 60/40 in bear trends | Trend-aware readings by regime |
Common mistakes
- Treating a 70 print as an automatic sell. In a healthy uptrend, RSI can hold above 70 for extended stretches. Fidelity’s own note on the 40–90 uptrend range says this out loud.
- Using RSI on illiquid or event-driven charts. On a name that gaps repeatedly, the smoothed averages get overwhelmed by outliers and RSI becomes noise.
- Cutler’s RSI vs Wilder’s RSI without knowing. Cutler’s variant uses a simple moving average instead of Wilder’s smoothing, which removes the “data-length dependency” but produces slightly different readings on the same series (Wikipedia: RSI variants). If two charting packages show different RSI values, this is often why.
- Mistaking RSI for Relative Strength (RS). RSI is an internal momentum oscillator on one series. Relative Strength (no “index”) is a ratio of one security’s price to another security’s price and is used to rank leaders vs laggards.
What RSI pairs well with
RSI is a momentum reading. On its own it tells you nothing about trend, support, or volume. Practitioners typically pair it with a longer-frame trend filter (a 50- or 200-day moving average) to decide whether to lean on the overbought or oversold side of the reading, and with volume or an unrelated indicator such as MACD to corroborate divergences before acting.
The most useful frame is probably the simplest: treat RSI as a description of how forcefully price has moved recently, not as a directional forecast. Combine it with your definition of trend, your position sizing, and your stops — and the 70/30 lines stop feeling like traps.
Sources & Further Reading
- J. Welles Wilder, New Concepts in Technical Trading Systems (Trend Research, 1978) — original publication of RSI.
- Fidelity Learning Center — RSI indicator guide.
- Wikipedia — Relative Strength Index (formula and Cutler variant).
- SEC Investor.gov — Technical Analysis glossary entry.
- Constance Brown, Technical Analysis for the Trading Professional (McGraw-Hill, 2nd ed. 2011) — regime-adjusted RSI thresholds.
Disclosure: This article is for informational purposes only and is not investment advice.